Average Headway
Calculate the average headway (time interval between successive vehicles), h = 3600 ÷ q, dividing 3600 seconds by the flow rate q (vehicles/h). The result, in seconds, is the average time between two consecutive vehicles passing a point. Headway is the inverse of flow: the higher the traffic volume, the shorter the intervals. It is a central concept of traffic flow theory, used in signal design, capacity analysis and car-following models. The smallest safe headway defines the maximum capacity of a lane. Enter the flow rate.
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Average headway (vehicle time gap)
Headway is the time interval between two consecutive vehicles passing the same point on a road — one of the most important microscopic concepts in traffic flow theory. On average it is simply the inverse of flow: h = 3600 ÷ q, with 3600 seconds per hour divided by the flow rate q (vehicles/h). The higher the traffic volume, the shorter the intervals: a road carrying 1200 veh/h has an average headway of 3 seconds. Headway is fundamental for several reasons. It sets capacity: the maximum throughput of a lane is limited by the shortest headway drivers keep safely (about 1.5–2 s at saturation, which yields roughly 1800–2000 veh/h per lane). It governs car-following models and the logic of adaptive cruise control systems. And it underpins traffic signal design, where the queue discharge headway during green determines how many vehicles clear per cycle. Very short headways signal risky driving (little reaction time), which is why a safe 'following distance' is often expressed in seconds of headway. Enter the flow rate.
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Average Vehicle Spacing
Calculate the average vehicle spacing, s = 1000 ÷ k, dividing 1000 metres by the traffic density k (vehicles/km). The result, in metres, is the average distance between the fronts of two consecutive vehicles in a traffic stream. Spacing is the inverse of density: congested roads have high density and small spacing; free-flowing roads have low density and large spacing. It is the spatial analogue of headway (which is temporal) and relates to speed by s = v·h. The smallest spacing, at jam density, equals the vehicle length plus the minimum gap. Enter the traffic density.
Equivalent Flow (PCE)
Calculate the equivalent flow in passenger car equivalents (PCE), q = Q_cars + Q_heavy × E, adding the car flow to the heavy-vehicle flow multiplied by the equivalence factor E (how many passenger cars each truck or bus equals in road occupancy — typically 1.5 to 3.0). The result, in PCE/h, converts a mixed traffic stream into an equivalent homogeneous one, allowing volumes to be compared and the capacity of roads with different traffic compositions to be computed. Heavy vehicles occupy more space and accelerate more slowly, especially on grades. Enter the car flow, the heavy-vehicle flow and the equivalence factor.
Saturation Flow
Calculate the saturation flow of a signalized approach, S = S₀ × N, multiplying the base saturation flow per lane S₀ (vehicles/h per lane, typically ~1800–1900) by the number of lanes N. The result, in vehicles/h, is the maximum rate of vehicles that can cross the stop line if the signal stayed green continuously and a queue existed — the queue discharge rate during green. It is a central parameter in signal design and intersection capacity, adjusted by lane width, grade, turning and parking factors. Enter the base saturation flow per lane and the number of lanes.
Peak Flow Rate (PHF)
Calculate the peak flow rate of a roadway, q = V ÷ PHF, dividing the hourly volume V (vehicles/h) by the peak hour factor PHF (between 0 and 1, the ratio of the hour's volume to four times the busiest 15-minute volume). The result, in vehicles/h, is the equivalent flow rate of the busiest 15-minute period — always greater than or equal to the hourly volume, since traffic does not arrive uniformly. It is the design flow used in capacity and level-of-service analysis by the HCM, since sizing by the hourly average would underestimate the peaks. Enter the hourly volume and the peak hour factor.
Space Mean Speed
Calculate the space mean speed of two vehicles by the harmonic mean, v_s = 2 ÷ (1/v₁ + 1/v₂), from the individual speeds v₁ and v₂. The result, in the same unit as the speeds, is the harmonic mean — not the arithmetic — which is the correct way to compute the mean speed of a traffic stream when observing a road section (average over space). Space mean speed is always less than or equal to the time mean speed (the arithmetic mean observed at a point), because it gives more weight to slow vehicles, which spend more time in the section. It is the speed used in the fundamental equation q = k·v. Enter the two speeds.
Number of Lanes Required
Calculate the number of lanes required on a road, N = V ÷ C_lane, dividing the design traffic volume V by the capacity of one lane C_lane (vehicles/h per lane). The result is the minimum number of lanes to serve the demand within capacity; in practice, always round up to the next integer. It is a basic sizing calculation in the geometric design of highways and urban roads, defining the cross-section from the predicted volume and the per-lane capacity (which depends on speed, road type and traffic conditions). Enter the traffic volume and the per-lane capacity.
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